fix(help): Disallow too many value names (unstable)

I can't think of a case for this or a way to render it.

Fixes clap-rs#2695

src/build/debug_asserts.rs

@@ -636,6 +636,18 @@ fn assert_arg(arg: &Arg) {
         );
     }
 
+    #[cfg(feature = "unstable-v4")]
+    {
+        let num_vals = arg.get_num_vals().unwrap_or(usize::MAX);
+        let num_val_names = arg.get_value_names().unwrap_or(&[]).len();
+        if num_vals < num_val_names {
+            panic!(
+                "Argument {}: Too many value names ({}) compared to number_of_values ({})",
+                arg.name, num_val_names, num_vals
+            );
+        }
+    }
+
     assert_arg_flags(arg);
 
     assert_defaults(arg, "default_value", arg.default_vals.iter().copied());

tests/builder/help.rs

@@ -2578,6 +2578,37 @@ OPTIONS:
     );
 }
 
+#[test]
+#[cfg(not(feature = "unstable-v4"))]
+fn too_many_value_names_panics() {
+    Command::new("test")
+        .arg(
+            Arg::new("foo")
+                .long("foo")
+                .required(true)
+                .takes_value(true)
+                .number_of_values(1)
+                .value_names(&["one", "two"]),
+        )
+        .debug_assert()
+}
+
+#[test]
+#[cfg(feature = "unstable-v4")]
+#[should_panic = "Argument foo: Too many value names (2) compared to number_of_values (1)"]
+fn too_many_value_names_panics() {
+    Command::new("test")
+        .arg(
+            Arg::new("foo")
+                .long("foo")
+                .required(true)
+                .takes_value(true)
+                .number_of_values(1)
+                .value_names(&["one", "two"]),
+        )
+        .debug_assert()
+}
+
 #[test]
 fn disabled_help_flag() {
     let res = Command::new("foo")